The utilization of renewable energy is significant in achieving sustainable energy supplies. In particular, the use of clean energy, including wind, water, solar, and tidal energy, as well as other renewable energy sources, could effectively relieve carbon emissions and become a promising alternative for the future power industry ^[1][2][1,2]. However, the instability and discontinuity of renewable energy, which is significantly affected by environment and climate conditions, requires highly efficient energy conversion and storage devices (ECSDs). Electrochemical ECSDs, such as secondary batteries, supercapacitors, and fuel cells, have been widely used in recent decades ^{[3][4][5][6][7][8]}[3,4,5,6,7,8]. Among them, rechargeable secondary batteries are the most effective electrochemical ECSD technologies, especially Li-ion batteries (LIBs), which are endowed with high energy density, long life cycles, environmental friendliness, and low self-discharge rates ^[9][10][9,10]. As a typical electrochemical ECSD, LIBs are used in electric vehicles, portable electronic devices, wearable energy devices, large-scale energy storage, and other fields ^[5][11][12][5,11,12]. However, the Li content in the Earth’s crust is only 0.0065%, and the need for lithium resources has also increased significantly with the large-scale use of LIBs, which has meant that it has gradually failed to meet the growing demand [2]. These reasons have further aggravated the rise in the cost of LIBs, making them gradually lose their advantages in low-speed electric vehicles, large-scale energy storage, and portable electronic devices.

Na and Li belong to the IA group, which is endowed with similar chemical and physical properties. In the crust, the Na content is 2.75%, more than 400 times that of Li, and it is rich and has a low cost [2]. Thus, SIBs with similar rocking-chair mechanisms based on Na⁺ reactions have attracted more attention [13]. In aprotic systems, the Na⁺/Na electrode potential is −2.93 V, close to that of Li⁺/Li (−3.05 V), which is conducive to improving battery voltage and energy density ^[14][15][16][14,15,16]. In addition, the desolvation energy of Na⁺ based on propylene carbonate solvent is 158.2 kJ/mol, obviously lower than that of Li⁺ (215.8 kJ/mol), which can form small solvated ions with faster diffusion kinetics in the battery reaction [17]. More importantly, the similar properties of Li and Na mean that the research and development of SIBs have many reference bases. For example, substituting many Li-containing cathodic materials with electrochemical activity with Na would lead to similar activity in SIBs, such as NaFePO₄, NaCo_1-x-yNi_xMn_yO₂, and other layered rock-salt structures [18]. The transition metal oxides, sulfides, and carbon materials, as well as silicon, have superior Li⁺ storage performance and also cause electrochemical activity in SIBs. This similar technical route has helped researchers make important progress in the research and development of SIBs, and the technologies have developed rapidly in a relatively short time ^{[1][18][19][20][21][22][23]}[1,18,19,20,21,22,23]. However, large differences in the radius of movable ions (1.02 and 0.76 Å for Na⁺ and Li⁺) result in slow kinetics, serious volume expansion in discharge–charge processes, and capacity decay, as well as the poor rate performance and cyclability of SIBs. Thus, it is the main bottleneck in developing long-life electrode materials with the stable intercalation/deintercalation of Na⁺, which seriously restricts the practical application of SIBs in the future [19].

MXenes are novel two-dimensional (2D) materials with excellent conductivity and low ion diffusion barriers that are widely used in energy storage and conversion, electrolysis, adsorption, and other surface-related fields ^{[24][25][26][27]}[24,25,26,27]. MXenes, such as Ti₃C₂, are fabricated by selectively etching Al layers in MAX Ti₃AlC₂ and have become one of the most important materials in electrochemical ESCDs. MXenes can mainly be obtained using two typical etching methods: wet chemical etching and molten salt etching. The resultant ultrathin 2D MXenes have large specific surface areas, high conductivity, rich active sites, and 2D ion transport channels [28], which can provide rich active sites for ion storage and enhanced reaction kinetics ^[27][28][27,28]. These superior properties endow them with highly applicable potential in SIBs, LIBs, Li-O₂ batteries, Li-S batteries, aqueous Zn-ion batteries, and supercapacitors ^{[29][30][31][32][33][34]}[29,30,31,32,33,34]. As electrode materials, their ultrathin structures have large specific areas, which are conducive to mass transport in the electrode, especially regarding the accessibility of electrolytes ^[35][36][35,36]. Simultaneously, with a typical layered structure, they have large layer spacing, which benefits the transfer of large ions in the interlayer and presents unobvious volume changes, ensuring structural stability. In addition, surface areas with functionalized group terminations could serve as active sites to adsorb or capture intermediators in discharge–charge processes. More importantly, this 2D structure has excellent electron conductivity, which causes fast electron transfer in the discharge–charge process and facilitates reaction kinetics ^[31][37][31,37]. In addition, MXenes have good adjustability, which can be converted from an accordion-like three-dimensional (3D) structure into nanodots, nanobelts, nanosheets, and porous 3D structures. They can also be functionalized using controllable surface modification via loading or the in situ generation strategy to construct new and highly efficient composites.

For MXenes, when used in batteries as active materials, their performance is limited by inevitable stacking, resulting in low capacity as well as poor durability and rate performances ^[26][38][26,38]. To effectively utilize MXenes, there has been extensive research carried out to optimize their electrochemical performance via surface structure functional modification, as well as the construction of heterostructures and hybrid structures. When MXenes are employed in compounding, their layered structures could provide more channels, and other materials adhering to MXenes or grown between MXene sheets could expand the layered space in MXenes and relieve the volume expansion in operation. Therefore, MXenes have good advantages in enhancing the stability and reversibility of sodium storage, and they can be developed into materials with high-capacity and good rate performance via reasonable structural design and combining their advantages with other sodium storage materials.

2. Structure and Synthesis of MXenes

2.1. Structure Characteristics

MXenes contain transition metal carbides and nitrides, marked as M_n+1X_nT_x. MXenes are obtained by etching MAX (M_n+1AX_n), where M is the Ti, Mo, Zr, and Cr elements; A is the Al, Si, Ga, and Ge elements; X is the C and N elements; and T is the functional groups on the MXene’s surface [39]. MXenes have a hexagonal, closely packed structure with an ABABAB type. However, due to the influence of element proportions, face-centered cubic structures also exists in M_3×2 and M₄X₃ with an ABCABC type [40]. Many MXene types have been explored and reported over the past decade, such as Ti₂C, Ti₃C₂, Ti₃CN, Ti₄N₃, V₂C, Cr₃TiC₂, Mo₂C, Mo₃ScC₂, Zr₃C₂, and Nb₂C ^{[39][40][41][42]}[39,40,41,42]. The structure of MXenes is highly dependent on the parent materials. Among these different MXenes, Ti₃C₂ has been widely investigated due to its accessibility, which is obtained from the hexagonal structure of Ti₃AlC₂, even with scale-up production [43]. With the removal of the Al layer from Ti₃AlC₂, the Ti₃C₂ phase presents high electron density at the Fermi level compared with the parent-phase Ti₃AlC₂, displaying metal conductivity. For MXenes, the majority of them have metal-like electron conductivity and hydrophilic natures due to conductive cores and rich surface-functionalized groups formed in the preparation process, which have good adjustability [44]. Benefitting from their unique structure, MXenes are considered promising 2D materials in electrochemical energy conversion and storage.

2.2. Controllable Synthesis

MXenes are usually prepared by selectively removing A from the M_n+1AX_n phase with Lewis acid (an HF solution, a mixing fluoride solution, or molten salt-containing halogen ions) [27]. After etching, an MXene changes from the block structure of its precursor to an accordion-like structure, and the surface of the MXene sheet will produce rich functional groups, such as F and OH, which also lay a structural foundation for the MXene’s easily modified properties. To obtain high-quality MXenes, the accordion-like structure needs to be further intercalated and stripped to form ultrathin MXene nanosheets so as to assure the advantages of having large surface areas and high conductivity. In an HF solution system, after etching, the MXene is chemically intercalated with organic molecules (such as DMSO and TMAOH) to form monolayer or few-layer MXene nanosheets [45]. As for the mixing fluoride solution method (HCl + LiF/NaF/KF), which is a more environmentally friendly and safer strategy, the combination of HCl and fluoride salts generates the HF solution and intercalation solvent, so the etching and delamination processes can be carried out simultaneously, which is for better controlling the MXene’s size and quality [46]. MXenes can also be obtained in molten salt, avoiding oxidation and surface defects, as well as producing adjustable functional groups. For example, Ti₃C₂Br₂ was successfully synthesized by halogen-compound-etching Ti₃AlC₂ in [47], while iodine in anhydrous acetonitrile benefited to oxygen-terminal Ti3C2Tx [48]. In addition, Sun et al. successfully realized MXenes exfoliated under fluoride-free action with LIB reactions by al-loying Li and Al layers at low potential, combined with the strategy of using a water-phase micro-explosion reaction, which is a relatively new and environmentally friendly method but may be limited by the output of products [49]. At present, this method is not widely used. Even so, the synthesis method, which used etchant, pH conditions, and treated temperatures, could affect the quality and productivity of MXenes, as well as the determination of the termination type and its distribution [44]. Therefore, in order to obtain an MXene with a specific structure and composition, it is very important to select appropriate preparation methods and conditions, which play an important role in the subsequent functionalization or further applications of MXenes. As new, ultrathin 2D materials, MXenes have many advantages and excellent characteristics, such as adjustable layer spacing, high conductivity, outstanding mechanical stability, rich polar functional groups, large surface areas, and modifiability. Therefore, they are widely used in electrochemical energy storage, such as SIBs, LIBs, Li-S batteries, Li-O₂ batteries, and supercapacitors ^{[31][32][34][49][50]}[31,32,34,49,50].

3. Anode Materials Based on MXenes and Their Composites

Sodium storage performance based on MXenes is restricted by inevitable stacking, low specific capacity, and high charge voltage ^[25][26][51][25,26,52]. To effectively utilize MXenes, many studies have been carried out. One of the main strategies is to effectively regulate the composition, structure, and surface characteristics of MXenes in order to achieve excellent sodium storage activities. Another important strategy is to construct MXene-based composite structures [1]. In composites, layered MXenes can provide more ion channels, and other sodium storage materials uniformly adhered to or grown on MXene sheets can expand the layered space. In particular, cheap, nonmetal materials such as Si, P, C, and low-cost oxides and sulfides have relatively high theoretical capacities ^{[51][52][53][54]}[52,53,54,55]. However, as shown in Figure 12, most of them result in low electron conductivity and serious volume expansion in discharge–charge processes, which results in poor rate performances and cyclability. Reducing the particle size and simultaneously introducing a high-conductivity substrate are effective ways of improving performance. Thus, the construction of MXene-based composites with sodium storage materials could improve specific capacity, cyclability, and rate performance via enhanced conductivity, stable intercalation/deintercalation, and promoted kinetics, which is the current focus of research.